Retina, ppi and scaling: understand it all

Density, not resolution

The resolution is the total number of pixels: 3840 × 2160 for 4K, 5120 × 2880 for 5K. The density is the number of pixels per inch of panel, abbreviated ppi. It is what determines perceived fineness, because it says how tightly packed the pixels are.

The calculation is simple: divide the diagonal in pixels by the diagonal in inches. A 4K on 27 inches gives 163 ppi; the same 4K on 32 inches drops to 139 ppi. Same pixels, larger surface, lower density.

The two balance points

Operating systems are designed around two densities, and anything between the two requires an extra calculation.

• About 110 ppi: the historic density. The interface displays at 100%, one interface pixel occupies one physical pixel. That is the case of a 34-inch at 3440 × 1440, at 109 ppi.
• About 220 ppi: Retina density. The interface is drawn twice as large, so every element occupies exactly four physical pixels — two across, two down. No approximation: the outlines land exactly on the grid. That is what gives text its characteristic crispness.

What happens in between

At 139 or 163 ppi, neither ideal case applies. The system then has to draw the interface at an intermediate size: it calculates a larger image and shrinks it, or applies a non-integer factor. The result stays very acceptable on modern applications, but the thin strokes of characters sometimes fall between two physical pixels. Hence that impression of slightly softened text, which does not jump out on a screenshot but is felt after a few hours of reading.

The 5K paradox

Here is what surprises people most: a 27-inch 5K, with its 14.7 million pixels, displays as many elements as an ordinary 2560 × 1440 monitor. You do not gain room: you gain four times as many pixels to draw the same thing. Icons do not shrink, windows do not multiply — everything simply becomes crisp.

That is exactly why resolution alone is not enough to compare two monitors. To gain surface you have to increase the size of the panel, not only the number of pixels: that is the subject of our comparison 5K or 6K.

macOS and Windows do not go about it the same way

macOS decides on its own: it detects the density and applies 2× scaling on a Retina panel, without asking you anything. You can then choose a display size for "more space" or "larger text", but the default resolution is the right one in the vast majority of cases.

Windows leaves it to you, in Settings › System › Display › Scale, and sometimes suggests a value that is not the best. On a 5K or 6K panel, choose 200%: it is the exact equivalent of Retina 2×. The details per model are in our Windows guide.

Viewing distance changes everything

Density is never judged in absolute terms. At 50 cm, the eye distinguishes the pixels of a monitor at 110 ppi; at a metre, much less. That is why a 55-inch 4K television looks crisp in a living room even though its density is derisory — you watch it from three metres.

For a desk monitor at 50-70 cm, the practical rule: below 120 ppi you can make out the grid, around 140-160 ppi the image is comfortable, from 200 ppi you can no longer see anything of the grid.

Three false ideas

• "4K is enough everywhere." On 32 inches, yes, it is even very coherent. On 27 inches with a Mac, 4K forces non-native scaling that you feel on text.
• "More pixels, more room." False at equal size: a 27-inch 5K panel displays the same workspace as a 1440p, only far crisper.
• "Scaling spoils the image." Not when it is integer. A 2× factor is mathematically exact: it is precisely the absence of approximation that makes Retina so crisp.